This invention relates to a process for sealing anodically produced oxide layers on aluminum or aluminum alloys by treatment with aqueous solutions at elevated temperatures to prevent the formation of troublesome aluminum hydroxide layers (sealing smut) on the surfaces thereof.
Anodically produced oxide layers are frequently applied to aluminum surfaces for the purpose of corrosion prevention. These oxide layers protect the aluminum surfaces against the effects of weathering and other corroding media. The anodic oxide layers are also applied to obtain a harder surface and, hence, increased resistance to wear of the aluminum. By virtue of the natural color of the oxide layers, coupled with the fact that in some cases they are easy to color, it is possible to obtain particularly decorative effects.
There are several known processes for applying anodic oxide layers to aluminum. For example, the oxide layers may be produced using direct current in solutions of sulfuric acid (direct current/sulfuric acid process).
The layers thus applied may be subsequently colored by immersion in solutions of a suitable dye or by an alternating-current treatment in an electrolyte solution containing metal salts. In many cases, however, solutions of organic acids such as, in particular, sulfophthalic acid or sulfanilic acid or a mixture of these acids with sulfuric acid are also frequently used for applying the oxide layers. These paticular processes are known as color anodizing processes.
However, the anodically applied oxide layers are not entirely satisfactory in regard to corrosion prevention because they have a porous structure. For this reason, the oxide layers have to be subsequently sealed. This sealing treatment is frequently carried out with hot or boiling water which seals the pores and hence considerably increases protection against corrosion.
However, in the sealing of anodically applied oxide layers, not only are the pores sealed, but a more or less thick, velvet-like coating known as sealing smut is also formed over the entire surface. This sealing smut consists of hydrated aluminum oxide and is not slip-resistant so that it spoils the decorative effect of the layer. In addition, it reduces bond strength when aluminum components are bonded and, through the enlarged effective surface, promotes subsequent soiling and corrosion. For these reasons, it has hitherto been necessary to remove the smut mechanically by hand or by chemical methods.
It is already known that sealed surfaces covered by sealing smut can be freed from the smut by treatment with a mineral acid. However, this process involves another treatment step and, in addition, requires very careful treatment with the mineral acid to prevent damage to the oxide layer.
It is also known that sealing smut can be prevented by carrying out sealing with solutions containing nickel acetate and lignin sulfonate. The disadvantage of this process lies inter alia in the yellowing of the oxide layers obtained under the effect of light. Finally, processes are also known in which, to prevent sealing smut, sealing is carried out in hot water in the presence of certain polyacrylates (German Patent No. 1,938,039) or certain dextrins (German Patent No. 1,944,452). These processes have proved to be very effective. In some cases, however, more or less visible polymer films can be formed on the surface, particularly with increasing molecular weight of the active substances used. These polymer films are undesirable. It has also been proposed to use hydroxy carboxylic acids, such as citric acid (German Patent No. 2,162,674), and also various phosphonic acids (German Patent No. 2,211,553) in small quantities as sealing smut inhibitors. Where these substances are used, however, it has been found that difficulties can arise through overdosage of the active substance, particularly in large, poorly circulated baths. This is because it is not always easy to maintain a concentration range in which sealing smut is safely prevented without adversely affecting the oxide layers. Although it was also found possible to reduce the danger of overdosage through the already known use of certain cycloaliphatic or aromatic polycarboxylic acids (German Patent Application No. 26 50 989), unintentional deviations from the optimal concentration range were still occasionally observed in practice.